structural analysis technique

8/2/2019 Structural Analysis Technique

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Structural analysis


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Preprocessing : Imp. aspects

1. Cross section of beam

Haunched beam ( varying cross-section ) :

Discretize the beam into many cross section with constant

cross-section.

2. Connection

Moment connection or shear connection

Releases in STAAD

3. Modeling of floor rigidity

Presence of a rigid diaphragm such as concrete slab.

Master slave method in STAAD to consider this effect.


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Preprocessing : Imp. aspects

1. Load & Load combination

Load distribution pattern : yield line theory or one way

distribution

2. Initial sizing of member

1. Concrete : Sizes as per architectural requirement or L/d

ratio given in IS-456

2. Steel : Slenderness ratio limits IS800 or approximate

member sizes that can carry the imposed load when it is

fixed or pinned on both ends.

3. Structure must be stable


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FINITE ELEMENT METHOD

1. Idealization of structure.

2. Discretization into elements

3. Develop stiffness matrices and force matrix.

4. Application of boundary condition

5. Solve for the displacement at the nodes

6. Solve for internal forces (shear or moment)


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IMPORTANT POINTS

1. Check the maximum deflection

L/300 or L/200 or refer ISCODE or Design basis

2. Optimize the structure

Changing structural arrangement or modifying the member


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Dynamic Models

1. Earth quake forces, Rotating machine ( e.g Pump,

Blower, Compressor) vibration.

2. Dynamic analysis methods- seismic coeff., responsespectrum,

3. Machine foundations Block type

4. Table Top machine foundation


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TYPES OF EARTHQUAKE ANALYSIS

LINEAR STATIC ANALYSIS PROCEDURE

This method is mainly suitable for regular buildings which respondprimarily within the elastic range.

Equivalent static load procedure or seismic coefficient method is

specified in most of the design codes.

A set of static loads are calculated based on the fundamental period of

the structure and the seismic conditions at site (zone, importance factor,

soil type).

The loads are distributed along the height of the building in a manner

consistent with the first mode shape.

This analysis is normally performed either by manual calculations or

using any analysis software.


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Response spectrum

1. Calculates time periods for a set of Modes

2. Calculation Sa/g for each mode utilizing time period anddamping for each mode.

3. Calculation of design horizontal acceleration Ak for differentmodes.

4. Calculates mode participation factor for different modes.

5. The peak lateral seismic force at each floor in each mode iscalculated.

6. All response quantities (shear force, deflection ) for eachmode are calculated.

7. The peak response quantities are then combined as permethod (CQC or SRSS or ABS) to get the final results


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Example: Five-Story Shear Frame

Earthquake analysis procedure

procedure described above is

implemented for the five- story

shear frame subjected to the El

Centro ground motion. Theresults presented are

accompanied by interpretive

comments that should assist us

in developing an understandingof the response behavior of

multistory buildings.


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System Properties

The lumped mass mj = m=100 kips/g at each floor

Lateral stiffness of each story = kj =k=31.54 kips/in.

Height of each story = 12 ft

Damping ratio for all natural modes= n =5%

11

121

121

121

12

1

1

1

1

1

kkmm


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Natural Mode Shapes

334.0

895.0

173.1

078.1

641.0

641.0

173.1

334.0

895.0

078.1

895.0

641.0

078.1

334.0

173.1

078.1

334.0

641.0

173.1

895.0

173.1

078.1

895.0

641.0

334.0

54321


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System Properties (Contd..)

Modal Properties

.sec2966.03383.04346.06852.00.2T

sec/rad1810.215708.184561.141703.91416.3w

ni

i


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Modal Expansion of m1


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Modal Static Responses


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Effective Modal Masses and Modal Heights

Effective modal masses Mn*= Vbn

st

Effective modal height hn*= Mbnst/ VbnstObsrve that Mn

* =5m= mj; hn* Mn

*=15 mh =hjmj


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Solution:

2m m

L

L

0

3m

Mb1st=2.069m

L

Mb1st=0.931 mL

1.218m

0.851m

0.851m

1.782m


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Solution

(Contd..)

851.0

782.1m

851.0

218.1m

0

3msss

851.0

782.1m

431.1

1

m

m3594.0ms

851.0

218.1m

097.2

1

m

m3406.0ms

594.0,m048.5M,m3L,Similarly

406.0m397.7

m3

M

L

397.7097.2

1

m

m3097.21mM

m30

1

m

m3097.21mL

431.1

1;

097.2

1

mL

EI

874.1w

;mL

EI6987.0w

21

222

111

222

1

11

1

T

11

T

11

21

32

31


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Other useful methods

1. Method of section : widely used in calculation of internalforces in combined footing

M = summation ( Pi * Xi ) where Pi is the pressure or axial load oncolumn

Treat the footing as a beam in equilibrium subjected to axialload /bending moment from the columns and a soil pressurefrom below.

Find out the moment and shear forces at some points alongthe assumed beam.


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Thank you

structural analysis technique

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